1. Field of the Invention
This invention relates to machines and methods for injection molding of parts using thermoplastic materials. It is intended primarily for use with structural foam and foaming additives, with or without an accumulator which stores the plastic material under pressure. However, it may also be used with most other thermoplastic materials where positive shut-off is desirable.
2. Description of the Prior Art
Injection molding machines have two platens which support the mold halves. If a plurality of shut-off bushings are required, for example with plural cavities or a large cavity which must be filled simultaneously at spaced points, the bushings may be mounted either on one of the platens or one of the mold halves. If on the platens, sprues are formed which must be later removed. Furthermore, since the bushings are of fixed length and their locations permanent, the machine cannot be used with molds having different injection points.
Mold-mounted bushings on the other hand are removeable with the mold and the machine is thus usable to mold different parts. However, for high pressure structural foam molding it has heretofore been feasible to use only expensive structural foam type machines, not less costly standard injection machines. This has been due at least in part to the inability to operate shut-off bushings in the confined space within the mold.
Gilmore Pat. No. 3,010,155 discloses an internally heated nozzle to maintain the plastic at the discharge end in a softened condition so as to avoid "freezing off" in the nozzle end which could be caused by coolant in the mold. The construction in this patent has a number of disadvantages as compared with the present invention, including the fact that there is no means for directly and positively shutting off the plastic flow at the nozzle tip when desired.
Driscoll et al U.S. Pat. No. 3,767,340 shows a mold sprue bushing which also has an internal heating element and discloses a slidable tip valve spring-urged towards its closed position. There is no means for positively controlling the shut-off independently of material pressure. This makes it difficult if not impossible to control the density of the molded product by varying the time that the nozzle is open. Furthermore, the Driscoll construction could result in shearing or degrading of the material due to molecular changes, caused by spring pressure which causes the material to be forced through a restricted orifice under high pressure as the springs initiate their shut-off action. These changes in the material could adversely affect the structural properties of the molded article in terms of strength, brittleness or color, especially where shear-sensitive materials, such as polycarbonates, are used.